Opacity monitors are used in power generating plants and in other applications wherein exhaust gases from some form of industrial process can release pollutants into the atmosphere. A typical opacity monitor includes a transmissometer which measures the transmittance of light through an optical medium to provide an indication of the opacity of exhaust gases flowing through an exhaust stack or duct; the opacity of the exhaust gas indicating the amount of particulate matter or pollutants in the exhaust gas.
One known form of monitor consists of an optical transceiver unit mounted on one side of a duct, and a reflector unit mounted on the other side. The transceiver unit contains a light source, a detector, and electronic circuitry, whereas the reflector unit houses a passive retroreflector. Another form of monitor uses a source on one side of a duct and a detector on the other side, the exhaust gas passing by a slotted pipe extending across the duct between the source and the detector.
In any type of opacity monitor, the monitoring system must be periodically calibrated to obtain a true zero reading and an accurate upscale range for an opacity reference. In the type of monitoring utilizing a source and a detector on opposite sides of the duct, zero calibration is obtained by manually inserting a calibration tube through the duct or by stopping the process while calibration is being effected. The calibration tube is awkward to handle and requires an operator to go to the monitoring location to effect calibration. Alternately, it may not be possible or practical to periodically stop the process for calibration. In the retroreflective system, calibration involves mechanically moving a mirror outside the source/detector housing window. Inherent disadvantages of this system include alignment problems in positioning the mirror, and the exposure of moving parts to a hostile environment.
Because of the foregoing, it has become desirable to develop an opacity monitor that can be readily and easily calibrated without stopping the process being monitored.